1. Field of the Invention
This invention is directed to polyimide products, especially polyimide sheet materials. More particularly, this invention is directed to a process for forming polyimide products of exceptional physical properties where the polyimide is formed by the cyclodehydration of a polyamide carboxylic acid, especially from a solution thereof in an aprotic solvent. More particularly, this invention is directed to such polyimide articles and to polyamide carboxylic acid solutions containing certain nitrogen compounds which provide the resultant polyimide with improved mechanical properties.
2. Discussion of the prior Art
Polyimide products, especially polyimide (PI) sheet materials and coatings, find many applications as a result of their interesting mechanical, thermal and electrical properties, being used, for example, as electrical insulating sheet materials for lining slots in electric motors.
For the manufacture of PI films, the starting material is, as a rule, a solution of a polyamide carboxylic acid (PACA) in polar solvents or solvent mixtures. The PACA is a still soluble polymeric intermediate of PI and can be obtained by the reaction of virtually equimolar proportions of diamine and dianhydride in the presence of aprotic polar solvents such as dimethyl formamide, dimethylacetamide or N-methylpyrrolidone (German Offenlegungsschrift No. 1,420,706). The insoluble polyimide itself is then obtained by evaporating the solvent after fabrication, followed by cyclodehydration of the PACA to the PI.
The cyclodehydration of the PACA to the PI may be performed by heat treatment alone at very high temperatures -- between 200.degree. and 400.degree. C, as a rule -- or it can performed at moderately elevated temperatures between 20.degree. and 80.degree. C in the presence of certain chemical agents (German Offenlegungsschrift No. 1,520,519). Carboxylic acid anhydrides, preferably acetic anhydrides or benzoic acid anhydride, have proven necessary for the chemical dehydration process. One disadvantage of the process of chemical cyclodehydration lies in the greater technological investment and the cost of the reagents, so that it has not yet achieved the importance of thermal cyclization.
Through the variation both of the diamine and of the dianhydride a plurality of structurally different polyimides can be manufactured. Probably the most interesting combination as regards characteristics and manufacturing cost is the polyimide based on pyromellitic acid dianhydride (PMDA) and diaminodiphenylether (DADE).
Although the properties of the polyimides are satisfactory, by and large, some of the mechanical characteristics, such as elongation and reflex folding frequency are not adequate for a number of purposes. The term "reflex folding frequency", as used herein, refers to the ability of a sheet of the polyimide to resist breakage when the same is folded back and forth on the same line. Thus, a high reflex folding frequency is determined by folding, at ambient temperature, a sheet of polyimide back and forth on the same line until the same fails, i.e., until there is a breakage at the line.
Additionally, the polyimide sheets generally have a good tensile strength ranging from 600 to 1,000 kp/cm.sup.2 (German Auslegeschrift No. 1,202,981). Elongations of these polyimide (PI) sheets when obtained from thermal cyclodehydration generally ranges from between 6 and 14% (see German Auslegeschrift No. 1,202,981, supra). However, the reflex folding frequency is usually only several tens of thousands, e.g., 50,000, and such low reflex folding frequency value is considered insufficient for many applications, especially in the electrical industry.
It is an object of the invention, therefore, to develop a process for the manufacture of PI sheet materials with substantially improved mechanical properties such as tensile strength, elongation and reflex folding frequency. This object has been achieved in a surprisingly simple manner, the process being one that is technically easy to perform.